This invention relates to the activation of chromium-containing olefin polymerization catalysts.
Supported chromium oxide catalysts can be used to prepare olefin polymers in a hydrocarbon solution to give a product having excellent characteristics from many standpoints. Supported chromium oxide catalysts can be used to prepare olefin polymers in a slurry system wherein the polymer is produced in the form of small particles of solid material suspended in a diluent. This process, frequently referred to as a particle-form process, has the advantage of being less complex. However, certain control operations which are easily carried out in the solution process are considerably more difficult in the particle-form process. For instance, in the solution process, control of the molecular weight can be effected by changing the temperature with lower molecular weight (higher melt flow) being obtained at the higher temperatures. However, in the slurry process, this technique is inherently limited since any effort to increase the melt flow to any appreciable extent by increasing temperature would cause the polymer to go into solution and thus destroy this slurry or particle-form process.
It is known in the art to utilize various techniques for altering the catalyst so as to give higher melt flow polymers but these techniques can cause a broadening in the molecular weight distribution. This is undesirable because many uses of high melt flow polymers, such as high speed extrusion and injection molding, require a narrow molecular weight distribution. For instance, in Hoff et al, U.S. Pat. No. 4,041,224 Aug. 9, 1977), it is disclosed to produce higher melt flow polymer by impregnating a titanium compound onto a chromium-containing catalyst, heating in an inert or reducing medium and thereafter reoxidizing. However, this technique is indicated to be ineffective for titanium-free catalyst systems and the presence of impregnated titanium, while not mentioned by the patent, results in a broad molecular weight distribution.